1. Field of the Invention
The present invention relates to diodes, in particular, relates to p-n junction diodes that are resistant to high voltage and high surge voltages.
2. Description of the Related Art
An internal combustion engine uses an ignition device. Such an ignition device consists of a primary coil and a secondary coil. A high voltage within a range of 10 kV to 30 kV (discharging voltage) is generated in the secondary coil when the supply of a predetermined voltage to the primary coil is interrupted. When the generated high voltage is supplied between electrodes of a spark plug in the ignition device, an electric discharge is generated to burn fuel in the internal combustion engine. Because the high voltage is generated in the secondary coil when the voltage supply to the primary coil of the ignition device is interrupted, a small voltage (ON-voltage) is also generated during the voltage supply to the primary coil. For this reason, there is a possible problem in which such an ON-voltage generated in the primary coil causes ignition combustion (ON-flying spark) at a timing earlier than, i.e. before a desired timing.
In patent document 1, Japanese patent laid open publication No. S58-48955, an additional diode is connected to the secondary coil in order to prevent the On-flying sparks. This additional diode is connected in a direction in which a discharging voltage allows a current to flow.
As shown in FIG. 2C, because an ON-voltage Va is generated in reverse polarity to a discharging voltage Vb, the additional diode can prevent the current generated by the ON-voltage, and this can prevent phenomenon of an ON-flying spark.
Because the ON-voltage has a high voltage within a range of 1 kV to 1.5 kV, it is necessary for the additional diode to have a high breakdown voltage (high withstand voltage) of not less than 2 kV. The diode 5x shown in FIG. 17A to FIG. 17F disclosed in the patent document 1 has a plurality of diode elements 90 connected in series. This structure of the diode 5x makes it possible to provide a high breakdown voltage (high withstand voltage) which is higher than a breakdown voltage of the single diode element 90.
A description will be given of a process of producing the diode 5x with reference to FIG. 17A to FIG. 17F.
FIG. 17A to FIG. 17F are schematic views showing a process of producing a conventional diode.
As shown in FIG. 17A, a deep p type semiconductor layer 90p is formed in an n-type semiconductor substrate 90n, and a diffusion wafer is prepared so that a plurality of the p-n junctions is formed on the entire surface of the diffusion wafer. The diode element 90 corresponds to the p-n junction formed on the surface of the diffusion wafer.
Next, as shown in FIG. 17B and FIG. 17C, the diffusion wafers are stacked to provide a lamination structure under vacuum by using solder in order to produce a lamination body 92 in which the diffusion wafers and the solder layers are alternately stacked.
Next, the produced lamination body 92 is cut by using a wire saw to produce a lamination part 93 shown in FIG. 17E.
Next, a cut surface of the lamination part 93 is etched by acid or alkali solution (end surface treatment). Next, lead wires 94 are connected to the lamination part 93, and molded by using resin material 95 to produce the diode 5x. 
However, the diode 5x has a large size because of having a plurality of the solder layers between the diffusion wafers.
In a usual planar diode, it is possible to increase a thickness of the p-type or n-type semiconductor layer in order to reduce its impurity concentration. It is accordingly possible to produce a single diode element having a high breakdown voltage without forming a lamination structure of the diode elements 90. However, when the impurity concentration of the diode is reduced, the breakdown voltage of the diode increases, and the surge voltage protection of the diode is reduced. This damages the diode element by a surge voltage generated when a spark plug discharges. Accordingly, because it is requested to enlarge the surface area of the semiconductor layer (to enlarge its chip size), it is difficult to prevent the entire size of the diode from increasing.
Various types of diodes, intended to have high breakdown voltage and high surge voltage protection, contain such a conventional problem previously described.